When Is It OK to Change the Original Ending?

How faithful should film adaptations be? The issue arises both with novels and with films that are remade: fans of the original are none-too-pleased to see the personality of beloved characters changed, settings or time periods moved, or -- horror of horrors! -- the ending changed. Watchmen ignited a mini-firestorm with the decision to alter the ending of the original graphic novel by Alan Moore and Dave Gibbons. On a somewhat smaller scale, faithful readers of Jodi Picoult's novel My Sister's Keeper are upset that the ending was changed for Nick Cassavetes' just-released movie version.

Author Picoult disclaims responsibility while trying to be diplomatic. "Yes, I know the ending is different," she writes on her official site. "Yes, I know some of you are very upset. I didn't change it. The author has no control over the movie, and it was hard for me to accept too. However, there's a great deal in the movie that I think is great, and I enjoyed watching it - and I hope you did too." She suggests that her fans let Warner Brothers know how they feel. As a point of interest, four out of five comments on my review for Cinematical have complained about the ending.

It seems foolish to try and establish a hard-and-fast rule that original endings should never be changed -- filmmakers should have the artistic right to exercise dramatic license when adapting a work to a different medium. Yet how often have film versions actually improved endings that they've changed?